Introduction
Coronavirus Disease 2019 (COVID-19) is the latest global health threat and, as in two preceding instances of the emergence of coronavirus respiratory disease, poses critical challenges for the public health, research, and medical communities (1, 2). Although the pathology of COVID-19 is now well described, the mechanisms underlying disease progression remain unclear. While a robust vaccination campaign and the further development of vaccines against SARS-CoV-2, the causal agent of COVID-19, are underway, a variety of investigational therapeutic approaches are also being explored (3). Dexamethasone, plitidepsin, and monoclonal antibody therapies, such as toclilizumab and eculizumab, have shown promise in lowering soluble inflammatory markers part of the cytokine storm and reducing severe outcomes in COVID-19 (4-7). Further elucidating effector molecules responsible for disease progression to determine effective interventions earlier in the course of the disease is needed in order to help design effective therapies to ameliorate disease manifestations and its complications (8-10).
The scope and severity of COVID-19 varies among those infected. Some patients exhibit no or minor flu-like symptoms and quick recovery, some have sustained fever and have persistent fatigue with a post-viral syndrome, while others experience serious lung involvement that requires hospitalization that may lead to death (11). Although the respiratory and the gastrointestinal system are initial targets for SARS-CoV-2, there clearly is a systemic nature to this disease in severe cases that may be driven by micro-emboli and inflammatory processes (12, 13). While follow up in natural history studies will likely uncover additional post-infection sequelae, the notable impairment in type-I interferon responses and rapid lymphopenia clearly plays a role in disease severity (14-16), highlighting the need for novel therapeutics that take into consideration the mechanism(s) of infection, viral replication, and effector pathways that lead to COVID-19 associated pathologies.
Inflammasome activation in peripheral immune cells and tissues was recently observed in COVID-19 patients and the level of inflammasome-derived products, including active caspase-1, associated with disease severity and poor outcomes (17). We recently reported that caspase-1 expression in lymphocytes and serum IL-18 levels are increased in liver transplant patients acutely ill with SARS-CoV-2 infection suggesting pyroptosis mechanisms may play role in severe COVID-19 (18). A recent study showed that SARS-CoV-2 infection of rhesus macaques led to an upregulation of caspase-1 molecular signature in peripheral blood cells as early as day 2 post-inoculation (19). Pyroptosis, also known as caspase-1-dependent cell death, is inherently inflammatory, triggered by various pathological stimuli (i.e. stroke, heart attack, cancer), crucial for controlling microbial infections (20-22), and characterized by rapid plasma-membrane rupture and the release of pro-inflammatory intracellular contents (23, 24), a marked contrast to the regulated death process of apoptosis (25). Insight into the complex activation and regulation of the inflammasome complex and the way in which COVID-19 intersects with this pathway is an area of significant investigation (26). Thus, strategies targeting the inflammasome/pyroptosis pathway upstream of the production of the effector cytokines may be a novel approach to reverse COVID-19 induced immune perturbations (27). Building on our previous findings, we sought to expand our analysis to investigate caspase-1 activity in SARS-CoV-2 infection, as well as the role of other caspases, including in red blood cells (RBCs) given the significance of COVID-19 associated coagulopathies (28, 29).